In many applications, it is useful or necessary to measure a distance between two locations. Towards that end, there are numerous known techniques for measuring distance. In some cases, distance is measured mechanically. For example, a wheel, ball or other rolling member can be rolled across a surface. Indexed counters coupled to the rolling member can then be used to determine the distance traveled. Such mechanical measuring systems have a number of disadvantages, however. Notably, such systems typically require physical contact with an object to which (or over which) distance is being measured. In some cases, such contact is not practical. Even when physical contact may not be a problem, the mechanical components of such a system may be relatively expensive and/or the source of other problems (e.g., dirt accumulation).
Distance can also be measured by reflection of energy (electromagnetic or sound) from an object. Such techniques avoid many of the problems with mechanical measuring systems, and offer numerous other advantages. In many of these techniques, a laser is used. Laser range-finding systems can be very accurate. However, known laser range finding systems have their own set of limitations.
One class of laser range finders includes “time-of-flight” (TOF) systems. In TOF systems, light from a laser is reflected from a target and received in a receptor. By measuring the time needed for light to travel from the laser to the target and then back to the receptor, the distance between the laser and the target can be calculated. TOF systems are commonly used for measuring relatively long distances (tens of meters or more). At closer ranges, the travel time for the light is extremely short (tens of picoseconds), and accurate measurement can be quite difficult without the use of expensive detection circuitry.
Another type of TOF system uses the round trip delay time of the laser light to form part of a variable frequency oscillator circuit. The oscillation frequency is then correlated to the distance. Still another TOF system uses a modulated beam and calculates time of flight indirectly by comparing the output beam with the reflected beam. These techniques suffer from limited measurement range, and temperature drift or calibration issues.
Another group of laser range finders includes triangulation-based systems. In these types of systems, light from a laser is reflected from a target and received by a receptor positioned a known distance from the laser emitter. Based on that known distance and the angle of the reflected light, the distance to the target can be trigonometrically calculated. Triangulation-based systems are commonly used for shorter ranges. As the measurement distances increase, the variation in the angle of reflected light becomes quite small. Accurately detecting such small angles can require expensive optics and detection circuitry.
Yet another type of laser range finder utilizes the self-mixing effect. In particular, a portion of light reflected from a target returns to an emitting laser and enters the emitting cavity. The reflected light mixes with light being generated in the cavity and affects the power output of the laser. The power output variations relate to the distance traveled by the light to the target and back. By measuring changes in the laser power output, distance can be determined. Self-mixing-based systems offer significant advantages over other types of laser range finding. Because the emitting laser is also used as a receptor, fewer components are needed. Self-mixing-based systems can also be very accurate. However, self-mixing-based systems also present a number of challenges. The signal generated by self-mixing can be quite noisy, and accurate measurement of the self-mixing effects on laser power output can require relatively complex and expensive circuits. For at least these reasons, such systems have generally not been used in many applications.